1. Field of the Invention
This invention relates to adhesive and more particularly, to pressure-sensitive silicon adhesives or silicone self-adhesives which are applicable to various supports without use of any primer and are cured at relative low temperatures. The invention also relates to a silicone self-adhesive tape or sheet using the above self-adhesive.
2. Description of the Prior Art
Known pressure-sensitive silicone adhesives are those which are obtained by condensation of dimethylpolysiloxane of a high degree of polymerization blocked with a hydroxyl group at ends of the molecular chain with copolymers comprised of SiO.sub.2 units and (CH.sub.3).sub.3 SiO.sub.0.5 units. The self-adhesive tapes or sheets obtained by coating these pressure-sensitive adhesives on film supports exhibit not only good self-adhesiveness and cohesiveness, but also good heat resistance, low temperature characteristics, electric characteristics and water proofing which are inherently possessed by silicones. However, when the supports are made of organic fluorine-containing resins, polyimide resins, polyamide-imide resins, polyamide resins, polyester resins, polyther imide resins, polyether ether ketone resins and the like, difficulties are encountered in that the self-adhesive is liable to transfer to an adherend because of the poor adhesion or anchoring force of the adhesive to the support.
For the fabrication of such self-adhesive tapes or sheets, it is the common practice that the support is subjected to chemical treatment with a sodium amide or naphthalene solution or a potassium bichromate solution or mechanical etching on the surface. Nevertheless, the anchoring force is not adequate.
Japanese Patent Publication Nos. 50-10615 and 60-11950 and Japanese Laid-open Patent Application No. 52-29831 propose methods in which a silicone primer layer is first formed on a support, either chemically or mechanically treated or untreated, and then a silicone self-adhesive top coating is applied on the primer layer.
These methods can improve the anchoring force but involve an additional step of the primer coating prior to the application of the self-adhesive layer. Further, the problem involved in existing self-adhesive tapes or sheets is not solved. When the tape or sheet is cut at a desired portion or in a desired form, the self-adhesive becomes stringed at the cut portion or spreads to outside, thus causing the surface of an adherend to be soiled. This problem cannot be solved by application of the primer.
The silicone self-adhesives conveniently adhere to any adherend and is applicable to a body to which non-silicone self-adhesives are unlikely to adhere. Accordingly, the silicone self-adhesives have been applied to supports including not only heat-resistant plastic films, but also supports of a lower heat resistance such as Japanese paper, cotton cloth, and films of polyvinyl chloride, polyethylene, cellulose acetate, regenerated cellulose and the like. However, the silicone self-adhesives are usually used in combination with a benzoyl peroxide vulcanizing agent. For imparting well-balanced tackiness, self-adhesiveness, cohesiveness and anchoring force to the resultant self-adhesive layer, it is necessary to cure the self-adhesive at temperatures not lower than 150.degree. C. The silicone self-adhesive tapes or sheets using supports of a low heat resistance cannot be baked at high temperatures, with lowerings of the cohesiveness and anchoring force. This leads to a difficulty in that the self-adhesive layer will transfer to the back surface of the tape or sheet or to the surface of an adherend, or cannot be readily cut in situ without involving any stringiness at the cut portion.